The present invention relates to divided wall exchange columns for heat and/or mass transfer processes. The invention has particular application in cryogenic air separation processes utilizing distillation, although it also may be used in other heat and/or mass transfer processes which use trays and packing (e.g., random or structured packing).
As used herein, the term “column” (or “exchange column”) means a distillation or fractionation column or zone, i.e., a column or zone where liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, such as by contacting of the vapor and liquid phases on packing elements or on a series of vertically-spaced trays or plates mounted within the column.
The term “column section” (or “section”) means a zone in a column filling all or part of a cross section of the column. The top or bottom of a particular section or zone ends at the liquid and vapor distributors (discussed below) respectively.
The term “packing” means solid or hollow bodies of predetermined size, shape, and configuration used as column internals to provide surface for the liquid to allow mass transfer at the liquid-vapor interface during countercurrent flow of two phases. Two broad classes of packings are “random” and “structured.”
“Random packing” means packing wherein individual members do not have any particular orientation relative to each other or to the column axis. Random packings are small, hollow structures with large surface area per unit volume that are loaded at random into a column.
“Structured packing” means packing wherein individual members have specific orientation relative to each other and to the column axis. Structured packings usually are made of thin metal foil, expanded metal, or woven wire screen stacked in layers or as spiral bindings; however, other materials of construction, such as plain sheet metal, may be used.
In processes such as distillation or direct contact cooling, it is advantageous to use structured packing to promote heat and mass transfer between counter-flowing liquid and vapor streams. Structured packing, when compared with random packing or trays, offers the benefits of higher efficiency for heat and mass transfer with lower pressure drop. It also has more predictable performance than random packing.
Cryogenic separation of air is carried out by passing liquid and vapor in countercurrent contact through a distillation column. A vapor phase of the mixture ascends with an ever increasing concentration of the more volatile components (e.g., nitrogen) while a liquid phase of the mixture descends with an ever increasing concentration of the less volatile components (e.g., oxygen).
Various packings or trays may be used to bring the liquid and gaseous phases of the mixture into contact to accomplish mass transfer between the phases. The use of packing for distillation is standard practice and has many advantages where pressure drop is important.
Initial presentation of liquid and vapor to the packing is usually made by means of distributors. A liquid distributor, the role of which is to irrigate the packing substantially uniformly with liquid, is located above the packing, while a vapor distributor, the role of which is to create substantially uniform vapor flow below the packing, is located below the packing.
There are several different types of liquid distributors typically used in air separation processes. One type, a pipe distributor, is comprised of an interconnecting network of closed pipes or ducts, typically comprising a central pipe or manifold and a number of arms or branches radiating from the central pipe. The arms are perforated to allow the liquid passing from the manifold and into the arms to be dripped or sprayed onto a packed bed below the pipe distributor. Upwardly flowing vapor passes easily in between each arm. Pipe distributors receive liquid from a separate liquid collector or an external source piped to the wall of the column.
Trough distributors compromise a collection of interconnecting open troughs having irrigation holes in the base to feed liquid to the packing below. One or more upper collection troughs, or a simple pot on top of the lower troughs feeds liquid to the lower troughs through a series or holes or overflowing notches. Vapor from the packing below passes upward between the liquid-containing troughs.
A divided wall column is in principle a simplification of a system of thermally coupled distillation columns. In divided wall columns, a dividing wall is located in the interior space of the column, such as shown in FIGS. 1 and 2. FIG. 1 illustrates a typical divided wall column 10 using a chord wall 12, while FIG. 2 illustrates another typical divided wall column 10 using an annular wall 14. The dividing wall generally is vertical.
The support of the dividing wall should not interfere with the installation of either the trays or the packing. The use of structured packing in a divided wall column requires that the liquid be uniformly fed over the top of the structured packing by the use of a liquid distributor. These requirements raise serious problems which must be addressed in the design and manufacture of divided wall columns.
For example, since two different mass transfer separations occur on either side of the dividing wall, which may have different operating pressures and temperatures, the dividing wall may have to withstand a pressure differential and/or a temperature differential across the dividing wall. The pressure differential can exert a significant force on the dividing wall, which must be countered by the mechanical design of the wall, and the temperature differential can give rise to an unwanted change in the distillation process adjacent to the dividing wall, which must be countered by some form of thermal resistance (insulation) between the two sides.
In the case of a chord wall design, the force of the pressure differential can be substantial. Prior art designs for countering such force are difficult and/or expensive to manufacture, often lead to an unacceptable loss in the column area available for distillation, or substantially interfere with the distillation process.
Another problem is that the prior art does not satisfactorily address how to design the layout of structured packing and/or trays in divided wall columns or, in the case of structured packing, how to design and arrange the liquid distributor.
U.S. Pat. No. 4,615,770 (Govind) and U.S. Pat. No. 4,681,661 (Govind) disclose dual interrelated distillation columns similar to the annular divided wall column illustrated in FIG. 2 herein. Neither patent addresses the need to increase the strength of the annular wall.
U.S. Pat. No. 5,709,780 (Ognisty, et al.) does recognize the need to minimize mechanical stresses on partition walls in an integrated distillation column having a partitioned stripping or absorption section due to a large pressure differential across the partitioning walls. The patent suggests that a curved or angled wall could be used rather than a substantially planar wall, which is preferred for ease of installation. It also suggests that mechanical stresses can be addressed by using a transverse rib or honeycomb type reinforcement of the partition walls or any trays in the partitioned section. It further suggests that the partition walls can have a laminate construction to establish an air gap or a layer of insulation between adjacent layers, apparently to help minimize stresses induced by temperature differentials.
U.S. Pat. No. 5,785,819 (Kaibel, et al.) discloses a distillation column separated in the middle by two walls with a gas space in between the two walls mounted in a longitudinal direction. The patent suggests the possibility of mounting spacers in the gas space between the two walls in order to increase the mechanical stability.
As discussed, the force on a chord wall can be significant due to pressure differential between the two sections. In addition, the chord geometry itself could require that the chord wall be supported even in cases with minimal or no pressure differential. The simplest way of dividing the column would be to use a flat sheet. However, although the thickness of the sheet can be increased, the increase in strength obtained is relatively poor, especially at large column diameters. Moreover, if the thickness of the chord wall is too dissimilar to that of the outer wall, there are complications associated with the welding of the chord wall to the column wall, as well as simply occupying a greater portion of the column area.
As discussed above, the prior art has attempted to avoid these problems by strengthening the chord wall in a way other than simply increasing the thickness, such as by using laminated or honeycomb walls, strengthening ribs, or even using the trays (if present) as stiffeners. True honeycomb walls and laminated walls are difficult and expensive to manufacture, although such walls do provide the benefit of higher thermal resistance if that is required.
For example, welding ribs to a wall tends to be expensive, since welding can distort a flat sheet, especially if the ribs must be attached only to one side. Also, if ribs are used in a packed column, the ribs may intrude into the structured packing and cause problems both with the installation of the packing and/or the distillation process. In the case of a preassembled stack of self-supporting trays being installed in a column, strengthening ribs on a dividing wall may intrude into the area where the tray stack is to be installed, leading to greater difficulty in installation; additionally, the trays may rest on the ribs when the column is lying on its side for manufacture or transportation and thereby be distorted, as well as cause problems with the distillation process.
It is desired to have a divided wall exchange column utilizing structured packing as a distillation device wherein the dividing wall is strengthened by strengthening means which do not cause a significant loss in distillation performance.
It is further desired to have a divided wall exchange column wherein the dividing wall is adequately strengthened to withstand pressure differentials and minimize temperature differentials across the dividing wall.
It is still further desired to have strengthening means to strengthen the dividing wall in a divided wall exchange column which means are relatively easy to design, manufacture, and install without excessive costs or expense.
It is still further desired to have a divided wall exchange column design which allows for use of a liquid distributor which is relatively easy to design and manufacture.
It is still further desired to have a divided wall exchange column in which the dividing wall can withstand the pressure differentials and minimize the temperature differentials during operation better than the prior art divided exchange columns.
It is still further desired to have an improved divided wall exchange column which overcomes many of the difficulties and disadvantages of the prior art to provide better and more advantageous results.
It also is further desired to have an improved cryogenic air separation plant having an improved divided wall exchange column which overcomes many of the difficulties and disadvantages of the prior art to provide better and more advantageous results.